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U.S. Government Documents (Utah Regional
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6-25-1990
The Daytime Wind in Valleys Adjacent to the Great
Salt Lake
G. L. Stone
D. Hoard
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Stone, G. L. and Hoard, D., "The Daytime Wind in Valleys Adjacent to the Great Salt Lake" (1990). All U.S. Government Documents
(Utah Regional Depository). Paper 96.
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APR 05 1990
Los Alamos Nallonal La~ralory
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aperaled I)y lI,e UnIVerSIty 01 Calolarnoa lor lhe Unlled
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Slates
De~rtmenl 01 Energy unCler conlract W-740S-ENG-36
LA-OR--90-720
DE90 008991
TITLE:
AUTHOR(S):
SUBMITTED TO:
THE DAYTIME WIND IN VALLEYS ADJACENT TO THE GREAT SALT LAKE
G. L. Stone, D. Hoard
Mountain Meteorology
Boulder, CO, June 25-29, 1990
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Los Alamos National Laboratory
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•
DISTRIBUTION CF TH!S DOcJtf
I
,
.,.
Daytime W'"md in Valleys Adjacent
to the Great Salt Lake
G. L. Stone
Los Alamos National Laboratory
Los Alamos, NM 87545
D.E.Hoard
Amparo Corporation
Santa Fe, NM 87504
1. Introduction
In 1986 Los Alamos National Laboratory was engaged by the US Army to study the meteorological aspects of emergency preparedness at several sites where
toxic materials are stored and handled. The project included a series of tracer and meteorological field experiments in the viciDity of the Tooele .fumy Depot, 60 km
southwest of Salt Lake City, Utah. These experiments
generated a large data set for validating numerical simulations and for empirical analyses of the local meteorology. This paper discusses the main characteristics of the
daytime, up-valley :Bow at the Utah site, including frequency of occurrence, horizontal and vertical structure,
and temporal evolution. Some parameters controlling
the variability in onset time for up-valley :Bow are identified, and an empirical forecasting scheme is discussed.
2. Site Description
Large topographic relief and the presence of two
lakes are the most striking features of the region (Fig. 1).
Valley and basin elevations range from 1284 m above sea
level (ASL) in the Great Salt Lake basin to about 1520
m ASL in Rush Valley. The Great Salt Lake covers
about 6200 km2 but has a mean depth of only 7 m.
Because it is shallow, surface water temperature stays
in phase with daily mean air temperature throughout
the year, and air-land differential heating occurs in all
seasons.
,\Ve will discuss the daytime, near-surface :Bow in
the Tooele and Rush valleys. These valleys lie between
the Stansbury Mountains to the west and the Oquirrh
Mountains to the east. Valley-to-peak relief generally
exceeds 1500 m. The :Boor of Rush Valley is a semiarid surface covered with a sparse cover of greasewoods,
,
I
•
changing to sagebrush on the alluvial fans that run
against the mountain slopes. Low, unsteady katabatic
wind velocities at night show that the valley is not well
drained.. The valley's high elevation (relative to adjacent valleys), its semiarid surface, and its basin-like te>
pography all strongly influence development of the daytime, up-valley wind.
Tooele Valley is lower in elevation and is open to
the north. High velocities for katabatic flow at night
show that it is well drained. The water table is close
to the surface; mud flats and agricultural fields cover a
large fraction of the northern. portion of the valley.
Toxic materials are stored in the South Area of
Rush Valley, tens of kilometers south of the population
centers of Tooele Valley. Therefore, understanding the
north-south exchange of air between these valley is of
vital interest to those concerned with emergency preparedness. South Mountain forms a 45O-m barrier to
north-south How; low points in this barrier are the narrow gap just north of Stockton and a higher pass on the
mountain's western fiank. The 700-mb pressure surface
is normally just below the major summits of the region.
We regard the wind at this level as representative of
large-scale flow and will refer to it as the gradient wind.
Sixty-six percent of the time during the summer months
the 700-mb wind direction is in the range of 190 0 to 290 0
and has a mean scalar value of 7.5 mls (Crutcher 1959).
3. Characteristics of the Daytime, Up-Valley
Wind
Below 700 mb, a thennally driven, topographically
controlled wind is often observed. This mesoscale circulation has a diurnal cycle forced by the differential
atmospheric heating caused by sloping terrain and the
contrasting thermal properties of land and water. The
earliest detailed account of this circulation was gi....en
by Hawkes (1947), in which he discussed the diurnal
wind systems in the Salt Lake and Dugway valleys and
in the Ogden area. Smidy (1972) investigated the seasonal variation of near-surface winds but limited the
study to winds in the Salt Lake Valley. More recently,
the Wasatch Front Regional Council (1980), motivated
by air poilution problems, conducted a study of the
wind system in the Great Salt Lake basin. And Astling
(1986), in a paper on mesoscale wind fields, discusses
the influence of the Great Salt Lake on circulation in
the Dugway Valley and along the southern shore of the
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•
lake. The Los Alamos study focused on the flow in
Tooele and Rush valleys, valleys that have not received
much attention m. previous investigations. In this paper
we discuss only the daytime phase of this circulation.
9.1 Wind Ro"u
The presence of a diurnal cycle in the near-surface
flow is very evident in the frequency wind roses for direction shown in Figs. 2a-b. These plots are based
on observations made 10 m above the ground at wellexposed sites. Statistics for stations 1 to 4 are based on
a January-t~August sample in 1987; those for station
32 are based on a June and August sample in 1986.
In our sample, the surface flow is up-valley at least
50% of the time in the late afternoon (Fig. 2&). Investigators of the Wasatch Front Regional Council study
(1980) concluded that local thermal forcing controlled
the near-surface flow about 80% of the time, but most
of their observations were made in and along the edge
of the Great Salt Lake basin. The N to NE wind directions at station 4 and the very high frequency of NNW
wind at station 32 suggest that the up-valley wind often
penetrates 45 km inland from the southern. shore of the
Great Salt Lake. For reference, the nighttime wind direction pattern is shown in Fig. 2b; katabatic flows are
evident throughout the region.
The frequent ENE ~d through the low passes separating Cedar Valley from Rush Valley, opposite to the
prevailing gradient wind, was an unexpected result and
is discussed by Stone and Hoard (1990).
9.~
Structure of the Daytime, Up- Valley Wind
An example of the horizontal distribution of fully
developed up-valley How is shown in Fig. 3. The gradient wind during this period was 8 mls out of the west.
Typical near-surface velocities in Rush Valley are 4 to
7 m/s. During our experiments, up-valley How was not
observed in the southern portion of Rush Valley, so the
maximum inland penetration of the flow is thought to
be about 55 km. There, up-valley winds are part of
a mesoscale circulation that has a horizontal scale of
approximately 100 km. The lower portion of this circulation begins far offshore, a fact well understood by
local wind surlers and sailboat enthusiasts.
Figure 4 shows vertical profiles of wind, temperature, and moisture in up-valley flow conditions in central
•
Rush Valley. In this example, a 300-m-deep up-valley
wind arrived just after sunset. The temperature profile
is particularly interesting because it shows the effects of
diabatic cooling near the surface. The 1700 MST sounding in Salt Lake Valley for this day showed onshore flow
to a depth of 500 m, and gradient winds were SSW at
3 m/s. Another set of late afternoon profiles shows a
500-m-deep up-valley flow in Rush Valley with onshore
flow to a depth of 700 m above Salt Lake City. Although the SOUDdings above Rush Valley and Salt Lake
City are not concurrent, evidence of a sloping frontal
inteIfa.ce (1:200 here) has been noted by others (e.g.,
Frizzola and Fisher 1963).
9.9 Et1oZ'ldion of tb.e Up- Valley Wind
The onset of up-valley flow at any given station is
usually clearly indicated by an abrupt change in wind
direction and an increase in wind speed. After onset, the
temperature trace often shows a slowdown or cessation
of daytime heating. Generally, however, the temperature record by itself is not a good indicator of onset
time.
When onset times are plotted as a function of distance south of the lake, we find that up-valley flow progresses southward, with the speed of a4vance increasing
with distance from the lake (Fig. 5). The :figure is based
on a small sample from July and August 1987, but the
sample is large enough to show that onset time at a
given station is quite variable. We also note that the
rate of advance of the wind reversal is variable.
Some of the variability in onset time appears to
be caused by changes in the soil moisture content in
Rush Valley. The solid lines in Fig. 5 show onset times
for 2 days when the soil moisture was 7% in the upper
lO-em layer, and the dashed lines show onset times for
2 days when the soil moisture was 15%. The gradient
wind was less than 8 mls and was mostly westerly on
all four days, so we tentatively conclude that changes in
the horizontal distribution of soil moisture can strongly
influence arrival time of the up-valley v..-ind through its
role in the surface energy balance. The sensible heat flux
density over the Rush Valley surface during the "dry"
period was observed to be approximately twice that during the "wet" period. Modeling studies by OokoucW et
al. (1984) show that mesoscale circulations are indeed
very sensitive to the horizontal distribution of available
soil moisture.
4
The advance of the up-valley wind behaves as a
front, at least in the sense that a line of convergence
moves southward. In Tooele Valley the rate of advance
is typically 1 mIs, much slower than wind speeds behind
the front, which are typically 3 to 5 m/s. Because the
front moves slowly and the transition (at a station) is
rapid, the frontal zone must be narrow; in Tooele Valley
we estimate it to be 1 to 2 km wide. By the time the
front reaches Rush Valley, its speed increases to 6 mIs,
which is comparable to normal wind speeds for the upvalley wind in that area.
As mentioned above, a temperature drop is usually
not observed with passage of the front. However, in
rare cases, such as late arrivals under the influence of
strong southerly gradient winds, a drop of 2 to 4°C is
observed. Normally, diabatic heating of the cool lake air
over the land is so rapid that it more than compensates
for temperature advection.
Probability density histograms are used to characterize the variability in onset time of the up-valley
wind at stations 1 and 2 (Fig. 6). The sample for each
station includes all cases from the January-to-August
dataset for which instrumentation was working reliably
and the wind direction reversal had the signature of local thermal forcing. Histograms for cessation times are
also shown; cessation of the up-valley wind marks the
onset of katabatic down-valley flow. These histograms
illustrate some interesting features:
(1) At station 2, the distributions are broader and
median onset and cessation times are later than those
at station 1. The distributions for onset time for upvalley flow cover almost the whole range of the daytime
heating cycle, therefore they would not be very helpful
for forecasting wind direction reversal times.
(2) Distributions for cessation time at both stations are much narrower. A tentative explanation for
this lies in the very different states of the valley atmospheres at sunrise and sunset. By late afternoon
the atmosphere is in a nearly adiabatic state. Any local cooling of air near sloping terrain quickly results in
the baroclinity that drives katabatic flow. Also, vertical
mixing subsides in late afternoon, which decouples the
surface flow from the variable influence of the gradient
windAt sunrise, on the other hand, katabatic flows have
concentrated the diabatic cooling that has occurred during the night. In poorly drained ,,-alleys and basins like
Rush Valley, deep ground-based temperature inversions
form, and these have to be destroyed before a low-level,
N-StempenUure~entcandevdop. Aho,duringthe
morning hours soil moisture availability strongly influences the sensible heat flux, and the in:8.uence of the
gradient wind strengthens as vertical mixing begins.
(3) It is not unusual for up-valley flow in Rush
Valley to continue for an hour or two after sunset, even
though dowu-valley Bow is under way in Tooele Valley.
At station 1, we observed a complete cycle of the
thermally forced wind on 70% of the days for which reliable data were avail.a.ble; the sample included data from
April to early August 1987. At station 2, we observed a
50% occurrence of the cycle; the sample included data
from late February to early August 1987.
Many of the features of the up-valley wind south
of the Great Salt Lake are simUar to those found in the
literature on lake and sea breezes. Wmd speed, depth,
frontal. advance speeds, inland penetration, and onset
time variability are all in the range found in the literature (see, for example, Simpson et al. 1977; Lyons 1972;
Keen and Lyon 1978; Moroz 1967; and. other references
cited in Atkinson 1981). At the Utah site one does not
see the effect of the Conolis force, which is often observed at other sites; topographical constraints on the
flow may account for t.his.
4. Forecasting the Onset of the Up-Valley Wmd
Many sites with the potential for accidental releases
of toxic materials are located in areas where diurnal
wind direction reversals occur because of local heating
and cooling. Accurate methods for forecasting the time
of these wind reversals could provide critical iDformation
for responding to an. emergency.
In principle, numerical simulations of the Bow should
provide the needed iDformation. Models, both analytical and numerical, have been used extensively to study
thermally driven mesoscale flows, and the results are
generally impressive (see references cited by Atkinson
1981). However, most of these investigations involve
case studies with comparisons with limited datasets, or
they involve parametric studies with no reference to observations. To our knowledge, Rye's (1989) study is
the only one that addresses the problem of forecasting
the onset time of a sea breeze in a routine application
(providing forecasts during the 1986-87 America's Cup
yacht races). Although the model showed skill in other
respects, Rye concluded that it showed almost no skill
in forecasting onset time. Others, for example, Estoque
et al (1976) and Moroz (1967), have found that simulations underestimate intensity and inland penetration of
the lake breezes. Underestimations also were found in
preJiminary model runs of the daytime Bow at the U tab
site (results unpublished).
The requirements for accurate routine simulations
of the How in Rush and Tooele valleys are formidable.
Site complexity requires a three-climensional calculation
over a large domain. Sharp temperature and velocity
gradients, as well as small but important topographic
features, require a small grid spacing, perhaps 4 km or
less. Initial and boundary conditions must be specified
in detail Specifically, parameters that affect the surface
energy balance, such as the spatial distributions of soil
moisture, snow, and clouds, would have to be specified.
Time variation of the large-scale How would also have
to be incorporated. The expense and complexity of providing this kind of information are perhaps as large a
..:hallenge as doing the calculation. At the present time,
in our opinion, numerical simulations, using standard
initialization schemes, cannot predict onset times with
sufficient accuracy for emergency response applications.
Results of an analysis of 7 months of data from
stations 1 and 2 suggest that an empirical approach to
the forecast problem may be more promising. Here we
adopt the view that most of the variance in the arrival
time caD. be explained by a few important, easily observed parameters.
Biggs and Graves (1962) adopted this approach in
a study of the lake breeze off of Lake Erie. They used dimensional arguments to identify important dimensionless numbers that govern the lake breeze phenomenon.
The ratio of the two most important numbers defined
a lake breeze index, (T = V 2 /6.T. Here, for simplicity,
we have included the constants in the index. V is wind
speed, irrespective of direction, 6.T is the difference between the maximum air temperature Ta and lake water
surface temperature. Both V and Ta are supposed to
be representative of conditions near the surface outside
the infiuence of the lake. The index is interpreted as a
ratio of inertial to buoyancy forces; when used in a diagnostic sense, the authors found that a critical value of
(T separates lake-breeze days from non-lake-breeze days
about 90% of the time. In other words, for a given
V, a lake breeze occurred ",·hen 6.T exceeded a critical
7
value of V2/u. Walsh (1974) considered the index concept within the context of his analytical model of the
sea breeze and found theoretical support for the linear
relationship between V2 and a critical aT.
Lyons (1972) applied the index concept to a study
of the lak-e breeze across the southwest coast of Lake
Michigan- He was also able to find a critical value for
(T that discriminated between lake-breeze and non-lakebreeze days about 90% of the time. :However, attempts
to correlate u with onset time of the lake breeze and
inland penetration were disappointing. The index apparently has no prognostic value. This is not surprising,
smce the element of time is not involved.
If the gradient wind is weak, the onset of the upvalley wind should depend mainly on the rate of heating of the valley atmospheres and the amount of heat
the system needs to develop the critical N-S temperature gradient. (In our notatio~ a positive N-S gradient
means that temperature increases toward the south.)
We have already noted (in Fig. 5) that changes in the
soil moisture in Rush Valley modulate the sensible heat
fiux and strongly infiuence onset time. We now consider
the amount of energy the system requires to develop the
baroclinity needed for the up-valley flow. The following
observations give some clues about this energy requirement.
(1) The diurnal temperature range in Rush Valley is larger than in Tooele Valley. At station 2, the
minimum temperature averages 5.2°C colder, and the
maximum temperature averages 3.4°C warmer than at
station 1.
In the daytime, a larger sensible heat fiux over the
Rush Valley's surface and the valley's smaller volumeto-area ratio contribute to the excess heating and development of a positive N-S temperature gradient. During
the night, cold air accumulates in the bottom of Rush
Valley because it is not well drained. Tooele Valley,
on the other hand, is well drained to the north, and
the nocturnal temperature inversions are less intense.
Therefore the presunrise N -5 temperature gradient is
always negative.
(2) During dry, light-wind conditions, the presunrise temperature difference is larger than average. in
such cases the N-S temperature gradient changes sign
later in the day, and the up-valley wind arrives late.
(3) When cloudy and/or windy conditions prevail.
during the night, the presumise N-S temperature difference is sm all In such cases, the temperature gradient
usually changes sign early in the morning, and the upvalley wind arrives early.
(4) The onset of up-valley How at station 2 occurs 1 to 3 hours after the N-S temperature gradient
becomes positive. Although this time delay is variable,
the time of the gradient's sign change appears to -have
some forecast value.
(5) Typically, the temperature in Rush Valley climbs
very rapidly in the early morning hours, exceeds that in
Tooele Valley about 4 hours after sunrise. The temperature gradient usually reaches its maximum value around
noon, stays relatively constant for several hours, and
then slowly diminishes during the late afternoon. The
average daytime N-S temperature gradient of 3.4°C/IO kIn:'
is an order of magnitude greater than typical gradients
across synoptic scale fronts. On days when a positive NS temperature gradient did not develop, up-valley winds
did not occur.
(6) There is no correlation between the magnitude
of the N-S temperature gradient and arrival time of the
up-valley wind, frontal speed, or its maximum velocity.
Observations 1 to 3 suggest that the energy deficit
of Rush Valley, relative to that in Tooele Valley, plays a
role in determining the onset of the up-valley wind. For
a given sensible heat Hux, the larger this relative deficit,
the longer it takes to develop the baroclinity required
to drive up-valley How.
Figure 7 is a scatter plot of onset time to of upvalley flow at station 2 vs the presunrise temperature
difference M' between sta.tions 1 and 2. Unfortunately
the dataset for which both stations were operating properly contains only 28 cases spanning April to early August. To the best of our knowledge, the solid dots, about
70% of the cases, represent the "normal" range of conditions. The open circles are cases in which the Rush
Valley surface was unusually wet or when strong gradient winds were known to exist. Had we assumed a linear
relation between to and tl.T, shown by the straight line,
and used M' as a predictor, we could have forecast onset time with a standard error of 57 min under "normal"
conditions; using the median onset time as a predictor,
the standard error would have been 98 min.
Theoretically we expect the relationship between
teaand t:a' to be nonlinear. We have been able to show
(in work in progress) that the time t it takes to develop
a positive, critical N-S temperature difference ~Tcrt is
given by the following expression:
where M is the presunrise difference (TT - Ta); cp
and Ii are average values of air specific heat and density; ~ is a ratio of valley top area to the volume of
air affected. by the heating; Q is the time rate of change
of t.he sensible heat flux density; and the subscripts R
and T refer to Rush and Tooele valleys, respectively. In
this preliminary work, we modeled. the presunrise potential temperature inversions as layers with constant lapse
rates, different in the two valleys. And we assumed a
constant slope to the sensible heat flux curve, an assumption usually justified during the :first part of the
heating cycle. We are currently investigating the utility
of incorporating this type of modeling into an empirically based forecasting scheme. Such a scheme would
account for both the energy required. to develop ~Tcrt
and the rate of energy deposition.
Our few observations regarding gradient wind effects are in accord with the consensus one finds in the
lakeisea breeze literature. Namely, onshore gradient
winds (up-valley at our site) result in earlier arrivals,
and offshore winds (down-valley at our site) delay the
arrival However, the number of cases with significant
along-valley gradient wind components is so few, we
were not able to study gradient wind effects, a deficiency our study shares with many other observational
studies of the lake and sea breezes.
5. Conclusions
We have used 7 months of data-weighted toward
the summer months-to characterize the daytime phase
of the diurnal mesoscale wind circulation in the valley south of the Great Salt Lake. At this topographically complex site, surface wind direction reversals occur about 70% of the time as katabatic down-valley flow
gives way to anabatic up-valley flow. Once established,
the up-valley flow penetrates about 50 km inland, where
10
I
it has a depth of 300 to 500 m and a near-surface velocity of 4 to 7 m/s. Driven by a N-S temperature gradient
an order of magnitude larger than synoptic values, the
wind advances southward during the day with frontal
speed increasing from 1 m/s near the lake to 6 mls near
the end of its range. Onset time at a particular station
is extremely variable; cessation time is less so.
In the context of emergency prepared-ness, accurate predictions of the onset time of up-valley flow could
be vital.. It is observed to be strongly infiuenced by
the sensible hP.8.t flux, presunrise temperature inversion
strength in the upper valley, and the gradient wind. Onset of the up-valley wind at inland sites is delayed when
strong inland temperature inversions exist, the soil is
wet (which reduces the sensible heating), the gradient
wind is offshore, or by a combination of these factors.
It may be possible to relate the onset time to an index
based on these parameters. A pre-jirninary analysis, accounting for only the temperature-inversion effects, suggests that it may rossible to predict the onset time . .\; th
a standard error sma!ler than 60 min at a site 25 km inland.
6. Acknowledgments
We thank J. Archuleta and B. Bourgeous for signmcant contnoutions to the field measl!l"ements 8l:ld
M. Williams for reviewing the manuscript. Kennecott
Copper, the Utah Sta.te Health Department, the National Weather Service, and Lawrence Livermore Laboratory all provided useful data from stations they operated. This work was done under the auspices of the US
Department of Energy with support provided by the US
Army Nuclear and Chemical Agency.
1. References
Astling, E. G., 1986: A study of mesoscale wind
field in mountainous terrain. US Army, Dugway Proving Ground, Dugway, UT 84022, TECOM Project No.
7-CO-R87-DPO-009.
Atkinson, B. W., 1981: Meso-Scale Atmospheric
Circv.la.tions. Academic Press, New York.
Biggs, W. G. and M. E. Graves, 1962: A lake breeze
index. 1. AppL Meteor. 1, 474-480.
11
•
Crutcher, H. L., 1959: Upper wind statistics charts
of the northern hemisphere. Issued by the Office of the
Chief of Naval Operations.
Estoque, M. A., J. Gross, and H. W. Lai, 1976:
A lake breeze over southern Lake Ontario. Mon. Wea.
Retl. 104, 386-396.
Frizzola, J. A., and E. L. Fisher, 1963: A series of
sea breeze o'bseI-ntions in the New York City area. J.
AppL Meteor. 2, 722-739.
Hawkes, H. B., 1947: Mountain and valley winds
- with special reference to the diurnal mountain winds
of the Great Salt Lake region. Ph.D. dissertation, Ohio
State University, 312 pp.
K~ C. S. and W. A. Lyons, 1978: Lake/land
breeze circulations on the western shore of Lake Michigan. J. AppL Meteor. 17, 1843-1857.
Lyons, W. A., 1972: The climatology of the Chicago
lake breeze. J. Appl. Meteor. 11, 1259-1270.
eo.
Moroz, W. J., 1967: A Ike breeze on the eastern
shore of Lake Michigan: o~tions and model. J.
Atm. Sci. 24, 337-355.
Ookouchi, Y., M.Segal, R. C. Kessler, and R. A.
Pielke, 1984: Evaluation of soil moisture effects on the
generation and modification of mesoscale circulations.
Mon. Wea. Reu. 112,2281-2292.
Rye, P. J., 1989: Evaluation of a simple numerical
model as a mesoscale weather forecasting tooL J. AppL
Meteor. 28, 1257-1270.
Simpson, J. E., D. A. Mansfield, and J. R. Milford,
1977: Inland penetration of sea-breeze fronts. Quart. J.
Roy. Meteor. Soc. 103,47-76.
Smidy, K. L, 1972: Diurnal and seasonal variation
of the surface wind in the Salt Lake Valley. M.S. thesis,
University of Utah.
Stone, G. L., and D. E. Hoard, 1990: An anomalous wind between valleys - its characteristics and a
proposed explanation. Preprints, Fifth Conference on
1?
Mountain Meteorology, June 25-29,1990, Boulder, CO.
Walsh, J. E., 1974: Sea breeze theory and application. 1. Atmo". Sci. 31, 2012-2026.
Wasatch Front Regional Council, 1980: Great Salt
Lake air basin wind study. Wasatch Front Regional
Council, Bountiful, UT 84010.
Figure captions
Fig. 1. Map showing the main topographical features and population centers of the study area.
Fig. 2. Frequency wind roses for direction for (a) late;t
afternoon and (b) early morning. The length of a spoke
represents the percentage of the time, within the interval shown, that the wind blows from the indicated
clirection. Ring interval is 5%.
Fig. 3. Fully developed up-valley flow. Vector
mean surface winds, 10 m above the ground, averaged
from 14 MST to 18 MST, August 4, 1987. Terrain contour interval is 275 m. Velocity scale in upper left corner.
Fig. 4. Vertical profiles of wind direction, wind
speed, water vapor mixing ratio, and temperature observed with a tethersonde in central Rush Valley, 20052046 MST, July 23, 1987.
Fig. 5. O&lSet time of the up-valley wind vs distance
south of the Great Salt Lake. Dashed lines show 2 days
when the Rush Valley surface was wet; solid lines show
2 days when the surface was dry.
Fig. 6. Probability density histograms for onset
time (te) and cessation time (tc) of the up-valley wind
at stations 1 and 2.
Fig. 7. Scatter plot of the onset time, \g, for the
up-valley wind at station 2 vs the temperature difference, tYI', between Tooele and Rush valleys just before sunrise. The straight line, drawn by eye, through
the solid points is thought to represent "normal" conditions. Open circles show effects of strong gradient winds
and/or high soil moisture in Rush Valley.
13
GREAT
SALT LAKE
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400
440
480
UTM COORDINATES (km East. Zone 12)
APPROXIMATE ELEVATION (m. ASL)
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20 km
t-=~-i
2500
1800
1500
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1600 - 2000 MST
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to (hours after sunrise)
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TEMPERATURE DIFFERENCE BETWEEN VALLEYS
~T
(0C)
0·
7.